An observer in motion relative to an object measures the length of that object along the direction of motion to be contracted when compared to the length measured by an observer at rest relative to the object.
- From a spacecraft, travelling at close to the speed of light, the lengths of objects in the outside universe appear to contract. The length of the spacecraft also appears to contract when observed from the outside universe.
Given an object moving with great speed, the distance traveled as seen by a stationary observer is L0 and the distance seen by the object is L′, which is contracted.
See also: Time Dilation.